The present invention relates to a method of forming a ceramic article, and particularly relates to a method of forming a ceramic engine component.
Silicon carbide based ceramics have received considerable attention for use as high performance structural materials where high strength, toughness retention, oxidation resistance, thermal shock resistance, and high thermal conductivity are demanded at temperatures approaching 1400xc2x0 C.
Ceramic articles made of silicon carbide are typically formed from particles of silicon carbide. One process for manufacturing particles of silicon carbide is the Acheson process. In this process, large cylinders of green solid carbon are laid out horizontally and packed in granular coke. The granular coke is then covered with granular sand. Large water-cooled electrodes are attached to the stack of the carbon cylinders, granular coke, and sand. A high current is passed through the stack to resistance heat the stack. During resistance heating, temperatures of 2000xc2x0 C.-3000xc2x0 C. are generated, which graphitizes the solid carbon. At the same time, a reaction between the sand and coke produces particulate silicon carbide.
Traditional processes for making a ceramic article from ceramic particles involve sintering or densifying the ceramic particles by heat treatment. Silicon carbide particles do not readily sinter due to limited diffusion between the particles of silicon carbide. Moreover, silicon carbide particles cannot be readily sintered into a discrete shape, making necessary the machining of the sintered material when close tolerances are called for. Many industrial applications, where silicon carbide particles are used, make use of a metallic phase, which acts as glue holding together the silicon carbide particles.
In some cases, ceramic articles of silicon carbide can be manufactured directly without forming intermediate ceramic particles. For example, U.S. Pat. Nos. 6,051,096 and 6,124,028, both to Nagle and Byrne, disclose a method of forming a ceramic article from a carbon precursor. In the method, wood is carbonized under controlled atmosphere and temperature conditions to produce a porous carbon product having the same cellular structure as the precursor wood. The porous carbonized wood is converted to a ceramic article by placing the porous carbonized wood in a sol gel of silica, drying the porous carbonized wood, and then heating the dried carbonized wood. Alternatively, the porous carbonized wood is placed on a bed of lump silicon within a graphite boat and heated until the silicon liquefies and reacts with the carbon to form silicon carbide. The ceramic articles formed by these methods have a silicon carbide honeycomb structure with silicon filling the pores.
The present invention relates to a method of making a ceramic article. In the method, a member is provided. The member includes a cellulose-based material. The cellulose-based material is carbonized to carbon. At least a portion of the member is covered with silica sand after carbonization. At least a portion of the carbon of the member, which has been carbonized, is converted to silicon carbide.
Another aspect of the present invention relates to a method of making a ceramic engine component. In the method, a cellulose-based member is provided. The member is heated in a substantially non-oxidizing atmosphere to carbonize the cellulose-based member to substantially carbon. The member is machined after carbonization to provide at least one surface of the member with a Ra surface texture with a maximum of about 1.0 xcexcm. The machined member is covered with silica sand. The amount of sand, upon reaction with the carbon in the member, is effective to convert substantially all of the carbon to silicon carbide. The machined member covered with the silica sand is heated in a substantially non-oxidizing atmosphere until essentially all of the carbon in the member reacts with the silica sand to form silicon carbide.